Conventionally, as a general-purpose antenna functioning as an on-vehicle AM/FM antenna, a telescope-shaped multi-stage rod antenna is constructed in such a manner that this rod antenna can be freely projected and also freely retracted, or withdrawn. Then, this rod antenna is set in such a manner that this physical projection length is approximately 1 m, and the physical length becomes approximately an 1/4 wavelength of the FM band signal under projection condition, so that the FM band signal can be resonated.
In the conventional antenna structure, the structure for projecting the rods, or the structure for retracting the rods is complex. Also, it is not preferable that such along antenna having a length of approximately 1 m is projected from a vehicle body.
Also, antenna elements with employment of helical coils are widely used, by which physical, or axial lengths can be sufficiently made short, as compared with the antenna effective lengths. Moreover, there are many cases that this antenna element is constituted by having flexibility so as to protect this antenna element from breakage. As one structural example of these conventional antenna elements, a helical coil is wound on an insulating resin rod having flexibility, an insulating resin pipe having thermal shrinkage is used to cover this wound helical coil, and then, the helical coil is fixed on the insulating resin rod by utilizing shrinkage of this pipe. Also, as an antenna element used in a microwave band, a helical coil having a relatively small winding diameter is embedded in an insulating resin rod having flexibility by way of the insert molding.
Therefore, the inventors have invented the following antenna as the technique capable of operating as an antenna for mounting on vehicle having a shorter projection length. That is, while the helical coil antenna was employed, the physical length of this helical coil antenna was selected to be approximately 15 cm, and also made shorter than an 1/4 wavelength of the FM band signal. Moreover, while the antenna effective length was selected to be approximately 1 m, the FM band signal could be resonated.
In accordance with this technique, since the physical length is made shorter, the length of the helical coil antenna which is projected from the vehicle body can also be made shorter, resulting in an improvement of the outer view. Moreover, the antenna characteristic with respect to the FM band signal could have the substantially same effect as that of the conventional antenna for mounting on vehicle, the projection length of which is approximately 1 m.
However, it is recognized that the antenna characteristic of the helical coil antenna with respect to the AM band signal is deteriorated. In this case, even in the conventional antenna whose projection length is approximately 1 m, the AM band signal is not resonated, but the output impedance of the antenna represents the capacitive characteristic. Also, the external load represents the capacitive characteristic, which is caused by the cable and the like connected to the antenna base terminal. The voltage of the signal received by the antenna is subdivided by both the output impedance of the antenna and the external load impedance such as the cable to constitute the essential antenna output. For example, assuming now that in the conventional antenna having the projection length of approximately 1 m, the capacitance of the antenna output impedance is 10 pF and also the capacitance of the external load impedance such as the cable is 100 pF, when these capacitances are converted into AC resistance values R1 and R2, the AC resistance value R1 becomes about 16 K.OMEGA. and the AC resistance value R2 becomes 1.6 K.OMEGA.. The antenna output would become approximately an 1/10 of the signal voltage VA. To improve this aspect, the capacitances of the cable and the like are reduced as small as possible, whereas the AC resistance value R2 of the external load impedance is increased as large as possible. However, in practical case, there are limitations in improvements of the cable and the like.
As a result, as previously explained with respect to the above-described technique, when the physical length of the helical coil antenna is made shorter, the output impedance of the antenna represents the larger capacitive characteristic. Thus, the AC resistance value R1 would exceed, for example, 50 K.OMEGA.. Also, since the projection length of this helical coil antenna is made shorter, the signal voltage would be lowered. As a consequence, the antenna output of the AM band signal is largely attenuated, namely should become considerably small, as compared with those of the conventional antenna.
Also, as to the conventional antenna element in which the helical coil is fixed on the insulating resin rod by using the insulating resin pipe having the thermal shrinkage characteristic, the winding diameter of the helical coil can be freely set, and thus, the freedom degree in design is high. However, there is the difficulty that when the helical coil antenna element is repeatedly bent, the coil pitch of this helical coil is shifted to be readily made unequal, and therefore, the antenna characteristic would be changed. Also, in such an antenna element that the helical coil is embedded into the insulating resin rod by way of insert molding, even when this antenna element is repeatedly bent, there is no difficulty that the coil pitch of this helical coil is shifted. However, when the winding diameter of this helical coil is increased so as to enlarge the diameter of the insulating resin rod, a "drop" would be readily produced during the resin molding, and thus, the ratio of defective products to good products would be increased in the manufacturing stage. As a result, this helical coil antenna element could be applied only to a helical coil antenna element having a relatively small winding diameter, for example, the winding diameter must be smaller than, or equal to 2.0 mm.